Improve UI if DSR ROMs are not found

[eswartz]

V9t9 has a usability issue in the absence of certain DSR ROMs. That is, the Device Selector allows you to enable support for things that reference missing ROMs, and the UI gives no hints about this. So you can try to set up disk stuff or RS232 stuff, which will never work (with the only clue being the italicized entries in the Setup dialog or the popup indicators about missing ROMs at time of access).

Instead, the Device selector and/or device buttons should be disabled or visually indicated to be in an error state if they cannot function due to missing ROMs.